Intrinsic charge dynamics in high-Tc AFeAs(O,F) superconductors

TL;DR

This study measures the in-plane optical conductivity of high-Tc iron-based superconductors, revealing a collective mode and a large superconducting gap, providing insights into their charge dynamics and pairing mechanisms.

Contribution

First direct measurement of optical conductivity in 1111 high-Tc iron oxypnictides, identifying a new collective mode and characterizing the superconducting gap.

Findings

Detection of a 2.5-meV collective mode suppressed at Tc

Observation of a large optical superconducting gap with 2D=kBTc=7

Strong coherence differences between crystal and film samples

Abstract

We report the first determination of the in-plane complex optical conductivity of 1111 high-Tc superconducting iron oxypnictide single crystals PrFeAsO0.6F0.35 and thin films SmFeAsO1-xFx by means of bulk-sensitive conventional and micro-focused infrared spectroscopy, ellipsometry, and time-domain THz transmission spectroscopy. A strong itinerant contribution is found to exhibit a dramatic difference in coherence between the crystal and the film. Using extensive temperature-dependent measurements of THz transmission we identify a previously undetected 2.5-meV collective mode in the optical conductivity of SmFeAs(O,F), which is strongly suppressed at Tc and experiences an anomalous T-linear softening and narrowing below T*=110K>>Tc. The suppression of the infrared absorption in the superconducting state reveals a large optical superconducting gap with a similar gap ratio 2D=kBTc=7 in both materials, indicating strong pairing.